text: The partial volume of a particular gas is a fraction of the total volume occupied by the gas mixture, with unchanged pressure and temperature. In gas mixtures, e.g. air, the partial volume allows focusing on one particular gas component, e.g. oxygen.
  It can be approximated both from partial pressure and molar fraction:
  * V is the partial volume of any individual gas component (X)
  * V is the total volume in gas mixture
  * P is the partial pressure of gas X
  * P is the total pressure in gas mixture
  * n is the amount of substance of a gas (X)
  * n is the total amount of substance in gas mixture